A wide variety of processes use radial or horizontal flow reactors to effect the contact of a compact bed of particulate matter with a fluid and in particular a gaseous stream. These processes include hydrocarbon conversion, adsorption and exhaust gas treatment. In most of these processes, contact of the particulate material with the fluid decreases the effectiveness of the particulate material in accomplishing its attendant function. In order to maintain the effectiveness of the process, systems have been developed whereby particulate material is semi-continuously withdrawn from the contacting zone and replaced by fresh particulate material so that the horizontal flow of fluid material will constantly contact a compact bed of particulate material having a required degree of effectiveness. Typical examples and arrangements for such systems can be found in U.S. Pat. Nos. 3,647,680, U.S.
Pat. No. 3,692,496 and U.S. Pat. No. 3,706,536 the contents of which are hereby incorporated by reference. A good example of the way in which moving bed apparatus has been used for the contacting of fluids and solids is found in the field of petroleum and petrochemical processes especially in the field of hydrocarbon conversion reactions. Many hydrocarbon conversion processes can also be effected with a system for continuously moving catalyst particles as a compact column under gravity flow through one or more reactors having a horizontal flow of reactants. One such process is the dehydrogenation of paraffins as shown in U.S. Pat. No. 3,978,150.
Another well-known hydrocarbon conversion process that uses a radial flow bed for the contact of solid catalyst particles with a vapor phase reactant stream is found in the reforming of naphtha boiling hydrocarbons. This process uses one or more reaction zones wherein the catalyst particles enter the top of the reactor, flow downwardly as a compact column under gravity flow and are transported out of the first reactor. In many cases, a second reactor is located either underneath or next to the first reactor. Catalyst particles again move through the second reactor as a compact column under gravity flow. After passing through the second reactor, the catalyst particles may pass through additional reaction zones before collection and transportation to a regeneration vessel for the restoration of the catalyst particles by the removal of coke and other hydrocarbon by-products that accumulate on the catalyst in the reaction zone.
In the reforming of hydrocarbons using the moving bed system, the reactants typically flow serially through the reaction zones. The reforming reaction is typically endothermic so the reactant stream is heated before each reaction zone to supply the necessary heat for the reaction. The reactants flow through each reaction zone in a substantially horizontal direction through the bed of catalyst. The catalyst particles in each reaction zone are typically retained between an inlet screen and an outlet screen that together form a vertical bed and allow the passage of vapor through the bed. In most cases the catalyst bed is arranged in an annular form so that the reactants flow radially through the catalyst bed.
Experience has shown that the horizontal flow of reactants through the bed of catalyst can interfere with the gravity flow removal of catalyst particles. This phenomenon is usually referred to as hang-up or pinning and it imposes a constraint on the design and operations reactors with a horizontal flow of reactants. Catalyst pinning occurs when the frictional forces between catalyst pills that resist the downward movement of the catalyst pills are greater than the gravitational forces acting to pull the catalyst pills downward. The frictional forces occur when the horizontally flow vapor passes through the catalyst bed. When pinning occurs, it traps catalyst particles against the outlet screen of the reactor bed and prevents the downward movement of the pinned catalyst particles. In a simple straight reactor bed, or an annular bed with an inward radial flow of vapors, pinning progresses from the face of the outlet screen and as the vapor flow through the reactor bed increases, it proceeds out to the outer surface of the bed at which point the bed is described as being 100% pinned. Once pinning has progressed to the outermost portion of the catalyst bed, a second phenomenon called void blowing begins. Void blowing describes the movement of the catalyst bed away from an inlet screen by the forces from the horizontal flow of vapor and the creation of a void between the inlet screen and an outer catalyst boundary. The existence of this void can allow catalyst particles to blow around or churn and create catalyst fines. Void blowing can also occur in an annular catalyst bed when vapor flows radially outward through the bed. With radially outward flow, void blowing occurs when the frictional forces between the catalyst pills are greater than the gravitational forces, or in other words, at about the same time as pinning would occur with a radially inward flow. Therefore, high vapor flow can cause void blowing in any type of radial or horizontal flow bed.
The production of fines can pose a number of problems in a continuous moving bed design. The presence of catalyst fines increases the pressure drop across the catalyst bed thereby further contributing to the pinning and void blowing problems, can lead to plugging in fine screen surfaces, contributes to greater erosion of the process equipment, and in the case of expense catalysts imposes a direct catalyst cost on the operation of the system. Further discussion of catalyst fines and the problems imposed thereby can be found in U.S. Pat. No. 3,825,116 which also describes an apparatus and method for fines removal.
Where possible, horizontal or radial flow reactors are designed and operated to avoid process conditions that will lead to pinning and void blowing. This is true in the case of moving bed and non-moving bed designs. Apparatus and methods of operation for avoiding or overcoming pinning and void blowing problems are shown in U.S. Pat. Nos. 4,135,886, 4,141,690 and 4,250,018.
Another problem that can effect radial flow reactors is fluidization of the upper particle bed surface and subsequent displacement or attrition of the catalyst particles. Fixed bed radial flow reactors commonly employ a variety of hold down methods to prevent fluidization of the top surface such as cover plates, inert packing material, or both. Typical cover plate and sealing arrangements for the top of fixed bed radial flow reactors are shown in U.S. Pat. Nos. 4,452,761 and 3,027,244. It has also been taught in a fixed bed arrangement to redirect a portion of the entering fluid from radial flow through the side of the bed to axial flow onto the top of the bed. This axial redirection of entering fluid provides containment of the upper bed surface as shown in U.S. Pat. No. 4,372,920. Moving bed reactors pose more difficulties since catalyst must be replaced while the upper surface of the bed remains in a sealed condition. High gas flows can be particularly disruptive and lead to fluidization and unwanted displacement of catalyst particles into other portions of the reactor internals. Complicated cover plate assemblies can stabilize the upper surface of the compact particle bed. U.S. Pat. No. 4,277,444 shows a system for confining an upper surface of a compact bed in a moving catalyst bed system. U.S. Pat. No. 5,130,106 shows a confining cover plate assembly that maintains downward pressure on the catalyst and prevents upward expansion of the bed.
Although the known cover plate assemblies can confine the upper bed surface and limit or prevent fluidization and churning of the catalyst, the transfer of catalyst in confined assemblies has resulted in occasional problems of cracking at weld seems due to thermal fatigue. The failures usually occur at a weld joint between a center screen and an imperforate screen section (hereinafter referred to as a "blank-off") designed to retain a sealing layer of catalyst above the compact catalyst bed.